Double-balanced mixer circuits are most commonly used in radio or television receivers. This type of mixer circuit is particularly suitable for low power frequency modulation (FM) receiver integrated circuits such as the Philips TDA 7000, as described in Netherlands Patent Application No. 82.00.959, and the Philips UAA 2080, as described in U.S. patent application Ser. No. 4,636,663. In both the Philips TDA 7000 and UAA 2080, the radio frequency (RF) signal from the antenna is applied to the input of a voltage-to-current converter that amplifies the RF signal and applies the result in phase opposition to the common-emitter terminal of the first and second differential pairs. A local oscillator (LO) signal is applied to the first and second common-base terminals and the RF signal is mixed down to create the intermediate frequency (IF) signal.
The dynamic range of such a receiver depends almost entirely upon the dynamic range of the mixer. The dynamic range of a receiver is characterized as the difference between the threshold RF signal level for intelligible reception and the maximum intermodulation product signal level for which reception is not disturbed. The difference is also known as intermodulation rejection and is expressed in decibels (dBs). In such circuits as the TDA 7000 and the UAA 2080, it has been found that the quality of the mixer is primarily determined by the quality of the voltage-to-current converter.
In the TDA 7000, the voltage-to-current converter comprises a differential amplifier. It has been found that the noise, and more importantly, the intermodulation rejection of such a common-emitter differential amplifier is sufficiently good enough to meet the requirements of a circuit when used, for example, in a car radio. The intermodulation interference is caused by the non-linearity of the differential amplifier of the voltage-to-current converter.
In the UAA 2080, the voltage-to-current converter comprises a balanced, common-base circuit. The balanced common-base circuit has better linearity and consequently exhibits better intermodulation rejection than a differential amplifier. It has been shown that, although greatly improved over the differential amplifier, the intermodulation behavior of the balanced common-base circuit still does not meet the stringent requirements of the types of receivers used, for example, in a portable selective call receiver. Thus, what is needed is a mixer circuit that meets the stringent requirements of selective call receivers.